Current and Deferred Income Tax Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Deferred Income Tax Assets And Liabilities
Income before income taxes	$ 94,113	$ 129,876	$ 115,588	$ 156,789
Current tax expense	31,218	46,642	40,027	57,772
Deferred tax expense	7,686	7,547	8,294	7,776
Total tax expense	$ 38,904	$ 54,189	$ 48,321	$ 65,548